[Jeri] uses light bulbs in an oscillator

Way back when [Ms Ellsworth] was a kid, she kept seeing the same circuit over and over again in her various op-amp books. It was a Wien bridge oscillator, a small circuit that outputs a sine wave with the help of a light bulb. Now that [Jeri] is much wiser, she decided to play around with this strange oscillator and found it’s actually pretty impressive for, you know, a light bulb.

The interesting portion of the Wien bridge is the gain portion of the circuit. It’s just a simple resistor divider, with a light bulb thrown in on one of its legs. When the current increases, this causes the light bulb to warm up (not enough to glow, though). When the temperature increases, the resistance in the light bulb increases, making the oscillator reach an equilibrium.

It’s a clever setup, but what about swapping out a resistor in place of the light bulb? In the video, [Jeri] tries just that, and it’s a mess. Where the light bulb circuit is amazingly stable with very, very low distortion, the resistor circuit looks like a disaster on the scope with harmonics everywhere.

A very cool build that would be perfect for an audio synth, but as [Jeri] says in her YouTube comments, “This doesn’t have enough distortion for indie bands.”

Using a light bulb in the exact same way to limit current for a battery charger. This is the absolute minimum possible battery charger and it will fry you or your battery over-easy if you forget about it, mess up the polarity or do something stupid. Unplug it before making any adjustments.

You can everything to make this charger, except the diode, from any hardware store or wal-mart in the country, so it’s a good emergency trick. For regular use or to fully charge, use a smart charger instead.

The key is using the positive resistance property of the lightbulb to make a really simple almost-constant current regulator: current increases->heat increases->resistance increases->current decreases. Ordinary resistors also has this property, and often they are used as such in even more circuits (nearly every pre/amplifier in the world), but it’s much less pronounced than in a small lightbulb. There are more modern parts that can be used in this context like PTC thermistors, FETs, Vactrols, etc. yet the lightbulb remains an ingenious way to accomplish an important task in the simpler possible way.

First: you do great work. Thanks for all the good info you’ve provided over the years. Second: This is off topic but is that the Tektronix dual domain device? I was given a short presentation of one of these at an EMC/EMI seminar recently and it seemed really cool. How useful have you found it to be?

i remember from the 80’s bose made a speaker with built in protection that i think used a light bulb like used in the dome light in car that would light up and even burn out like a fuse if you tried to push too much power say connect directly to ac line or turned up the volume too high or if the amplifier driver shorted.

FWIW There’s a wonderful article in “Analog Circuit Design” by Jim Williams (who’s also the editor) entitled “Max Wein, Mr. Hewlett and a Rainy Sunday Afternoon” which narrates his efforts at removing the lamp which resulted in THD below the noise floor of the distortion analyzer (0.0003%). But even his first iteration w/ a lamp was only 0.0025% THD.

A diode works just fine, there is nothing magic about the nonlinearities of a light bulb. see http://cnx.org/content/m32489/1.1/ for a simple example, you just need some kind of nonlinearity that causes the loop gain to drop before the opamp starts to clip.

However, there is a misunderstanding about the role of the light bulb in reducing distortion. The light bulb serves as a kind of automatic gain control, changing its resistance so the circuit achieves unity gain (and thus stability) near a desired output amplitude. Compare this with other oscillators that use non-linear gain (i.e. distortion) to limit output and you see why the Wien bridge produces a good signal. The light bulb also allows for reasonably quick start up by providing greater than unity gain when it is cold.

Now, notice that the light bulb changes its resistance slowly and does not appreciably affect distortion during any particular cycle after its temperature has stabilized. If it changed at that time scale, the output signal would be distorted. Replacing the bulb with an equivalent ideal resistor after stability is reached would improve signal quality for a short time, but output amplitude would soon drift, either extinguishing or distorting the signal.

So why did Jeri’s video show so much distortion when the resistor was swapped in? Mostly because the op amp was clipping high output levels. But if you look closely, it still appears that the distortion was a little higher than the light bulb version even during those times the output level was moderate. This is not because of the light bulb’s magical resistance, rather it must be a consequence of non-linearity in the variable resistor. Keep in mind that the resistor swapped into the circuit was dissipating just as much energy as the light bulb, and was undoubtedly heating up. This can be easily remedied by swapping out both the light bulb and the negative feedback resistor for new resistors with proportionally higher values, aiming for perhaps a few milliamps of current.

Yeah, I mean, what is he remembered for,.. the phonograph (almost gone!), the incandescent lamp (almost gone!), I think he experimented with a wire “tape” recorder, (almost gone!), but his experiments in food replication, teleportation, and warp drive will come to fruition soon, so future generations will rediscover him B^)…

Yea,yea ;) ; the ban on incandescent light bulbs that never actually existed. The effective ban due to the performance requirements was actually short lived, as we can purchase 10 W. lamps that pass muster. Don’t forget to ;look at the motor vehicles far incandescent lamps for hacking purposes.

when we where studying bridges at tech school I thought I’d build projects to measure the value of unknown capacitors & inductors. Have yet to do so. I guess I didn’t that as much as I thought I would :) Icould say I have been waiting for inexpensive uproccesor dev. boards to include a digital readout. :)